1. Field of the Invention
The invention concerns the selection of one station from a set of stations dialoging with a main station to which they are connected by a multiplex link consisting of at least one multiplex line for each communication direction, the dialog consisting in an exchange of messages transmitted in a reserved time slot of each frame of the multiplex line which carries information in coded pulse form contained in the time slots of a frame. As a rule, the frame has duration of 125 microseconds and is divided into 32 time intervals referred to as time slots.
2. Description of the Prior Art
In a centralized type communication network, the stations are terminal units connected to a certain number of subscriber lines and to a control unit equipped with a main station which consults said stations, this control unit being itself connected to the telecommunication exchange by a multiplex link.
The stations dialog with the main station by means of a reserved time slot in the frames of the multiplex lines, and the time slot reserved for dialog with the main station may be any of the 32 time slots of the frame. It is not possible for more than one station to dialog with the main station at a time. It is therefore necessary for the stations to share the reserved time slot and to use it in turn.
There are two methods for dynamic sharing of a reserved time slot, the competitive method and the elective method.
In the competitive method of dynamic sharing each station sends its messages at will without making any request beforehand and without concerning itself with the other stations. The major disadvantage consists in the resulting collision of messages, which are thus degraded. This phenomenon entails the need for repetitions which increase with the load and the number of stations, resulting in a lack of efficiency through a severe limitation on the bit rate. To partially eliminate such collisions, while retaining the competitive principle, use is made either of discrete time intervals, whereby each sending station must wait for the beginning of a particular time interval in order to send a message, or the listening method which is based on two mechanisms: detection of a free time slot (listen before send) and detection of collisions (listen during send). These two solutions require additional hardware and software at each station and make it difficult to monitor the activity of each station.
In the elective method, a station may be elected either by consultation or by centralized selection. In the consultation method, all stations are interrogated in a fixed order. When a station which is ready to send is encountered, it is elected and the consultation stops. Once the message has been sent consultation resumes. Disadvantages of this method include a scanning time which increases with the number of stations and a service order which bears no relationship to the order of requests for service.
In the centralized election method an arbiter stores requests from the various stations and carries out the election. Disadvantages of this method consist in the complexity of the arbiter and in the fact that the requests are themselves messages which must be sent in a dedicated time slot.
The object of the invention is to select a station in a way which, by comparison with the prior art methods, makes possible a significant saving in terms of hardware in each station while retaining good communication performance, that is to say by limiting the effect of the number of stations on the bit rate and by taking into account the order of requests.